Stamp with drainage channels for transferring a pattern in the presence of a third medium

ABSTRACT

A method for transferring a pattern from an elastic stamp to a substrate in the presence of a third medium is described. A proximity contact is achieved between the stamp and the substrate. A layer of the third medium between the stamp and the substrate is controlled to a predetermined thickness. Stamps for carrying out this method are also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/249,322, filed Oct. 10, 2008, now U.S. Pat. No. 7,891,295, which is acontinuation of U.S. patent application Ser. No. 10/527,277, filed Aug.4, 2005, now U.S. Pat. No. 7,434,512, which is a U.S. National Stage ofApplication No. PCT/IB2003/03834, filed on Aug. 28, 2003, which claimspriority to European Patent Application No. 02405777.0, filed Sep. 9,2002. Priority under 35 U.S.C. §119(a) and 35 U.S.C. §365(b) is claimed,and all the benefits accruing therefrom under 35 U.S.C. §119, thecontents of which in its entirety are herein incorporated by reference.

BACKGROUND

The present invention generally relates to printing and particularlyrelates to methods and stamps for transferring patterns to a substratein the presence of a third medium.

Printing thin layers of ink or other material from a patterned surfaceis well known in the printing industry. Printing processes wereoriginally developed for the exchange and storage of information adaptedto human vision. This typically requires pattern and overlay accuraciesdown to 20 μm for acceptable reproduction. Printing processes have beenused for other forms of patterning. For example, gravure offset printinghas been used to make 50-μm-wide conductor lines on ceramic substratesand to pattern thin-film transistors in low cost display devices. Offsetprinting has been used for fabrication of capacitors and metal conductorlines as narrow as 25 μm. Additionally, printed circuit board andintegrated circuit packaging are popular applications of screen printingin the electronics industry. See, for example, B. Michel et al., IBM J.Res. Develop. 45, 697 (2001) and references therein.

Another conventional printing process is known as flexography. Inflexography, a viscous ink is printed onto permeable materials such asporous paper, permeable plastic, and the like. Flexography is a rotaryprinting method involving resilient relief image plates to print imageson materials that are difficult to print on with offset or gravureprocesses. Examples of such materials include cardboard, plastic filmsand substrates. Flexography is therefore used widely in packaging.Usually, the viscous ink prevents direct contact of the stamp with thesubstrate because it cannot be displaced quickly enough during fastprinting operations. Transfer of a thick layer of ink is usuallydesired. However, this prevents replication of small feature sizes,typically smaller than 20 μm. See, for example, H. Kipphan, “Handbuchder Printmedien”, Springer Berlin, 2000 and J. M. Adams, D. D. Faux, andJ. J. Rieber, “Printing Technology 4th Ed.”, Delamare Publishers,Albany, N.Y.

Micro contact printing uses a similar stamp to that used in flexography,but typically transfers a monolayer of ink onto an impermeable surface.A more general process called soft lithography has been applied toprinting thiols and other chemicals onto a range of surfaces. Typically,the chemicals are first applied to the stamp as solutions in a volatilesolvent or via a contact inker pad. After inking and drying, moleculesin the bulk and surface of the stamp are in a “dry” state. The moleculesare transferred by mechanical contact. The stamp is typically formedfrom poly-(dimethyl)siloxane (PDMS). See, for example, B. Michel et al.“Printing meets lithography”, IBM, J. Res. Develop. 45 (5), 697 (2001)).

Micro contact processing, soft lithography, and flexography involvelocally defined, intimate contact without voids between stamp andsubstrate. This is generally known as conformal contact. Conformalcontact comprises macroscopic adaptation to the shape of the substrateand microscopic adaptation of a soft polymer layer to a rough surface.

Micro array technology is expected to accelerate genetic analysis. Microarrays are miniature arrays of gene fragments or proteins attached to ordeposited on glass chips. These so-called “biochips” are useful inexamining gene activity and identifying gene mutations. A hybridizationreaction is typically used between sequences on the micro array and afluorescent sample. In a similar manner, protein markers, viruses, andprotein expression profiles can be detected via protein specific captureagents. After reaction, the chip is read with fluorescence detectors.The intensity of fluorescing spots on the chip is quantified. The demandfor micro arrays and techniques for fabricating micro arrays isincreasing. Conventional methods for patterning biological moleculesonto biochips are described, for example, in M. Schena, “Micro arrayBiochip Technology”, Eaton Publishing, Natick Mass., (2000). In a firstconventional method, a surface is treated with compounds in a sequentialmanner by: pipetting with a pipetting robot or capillary printing;dispensing droplets with an ink jet; or, patterning with a pin spotter.In a second conventional method, a surface is patterned with moleculesin parallel thus reducing manufacturing cost. Microfluidic networks,capillary array printing, or micro contact processing can be employed inimplementation of the second method.

The printing of biological molecules and water soluble catalysts byconventional techniques does not always work, is difficult to reproduce,and results are variable. Repetitively creating homogeneous prints withhigh yield over large areas is very difficult, particularly if themolecules require permanent hydration. See, for example, A. Bernard etal., “Micro contact Printing of Proteins”, Adv. Mater. 2000 (12), 1067(2000). Many biological molecules require at least partial hydration.Also, many biological processes operate only when there is liquid toprovide mobility. When molecules are to selectively perform chemicalreactions on a surface in a patterned fashion, it is desirable to fixthe molecules in place to avoid blurring the pattern by spreading. Incatalytic printing therefore, it would be desirable to tether moleculesso that they can reach the surface only where desired. Limited mobilityshould be permitted so that molecules can function effectively withoutescaping. Biological molecules preferably encounter the substrate whileimmersed in a layer of water to permit a chemisorption reaction. Becausechemisorption reactions of proteins are not selective and many potentialanchoring groups may be present on the substrate, mobility requirementsare lower. For molecule-molecule interactions, control over hydration isdesirable. One way to prevent drying without immersion in water is towork in saturated air. In many printing operations, this is helpful.However, the humidity level is difficult to regulate. Molecules caninteract creating adhesion detectable with an adhesion sensor asdescribed, for example, in EP 0 962 759 A1. For example, an antibody andits matching antigen may interact. Similarly, a DNA oligomer mayhybridize with its complementary oligomer.

Other printing technologies include Ultra Violet (UV) lithography orUV-molding. In such techniques, a patterned glass master is pressed intoa liquid prepolymer. The prepolymer is then cured and solidified byexposure to UV light. See, for example, M. Colburn et al., “Patterningnonflat substrates with a low pressure, room temperatureimprint-process”, J. Vac. Sci., Technol. B. 6, 2161 (2001). On release,the pattern formed in the polymer is a replica of the master. However,it is difficult to displace such a polymer on large areas to achieve apattern with acceptable definition. There is usually a residual layerleft. Use of an identically patterned elastomeric stamp in place ofglass provides similar replication except for two differences, asfollows. Experiment indicates that in protruding areas of the stamp,where the polymer was to be displaced down to the surface, localizeddome-like protrusions of trapped material were discovered. Secondly,variation was observed in the thickness of features molded from therecesses in the stamp Typically, the thickness of each feature wassmaller in its center. The depth of depression was proportional to theload applied to the stamp. See, for example, Bietsch and Michel,“Conformal contact and pattern stability of stamps used for softlithography”, J. Appl. Phys. 88, 4310 (2000); Johnson, “ContactMechanics”, Cambridge University Press, Cambridge (1985); and S. P.Timoshenko and J. N. Goodier, “Theory of Elasticity”, Mc-Graw-Hill, NewYork). Formulae for the displacement of liquids can be derived fromlubrication theory. See, for example, A. Cameron, “Basic LubricationTheory” Wiley, New York (1981)).

Hydrogels are used in gel electrophoresis. Because hydrogels areflexible, they are also used as stamp materials for printing ofbiological molecules. See, for example, D. Brett et al., Langmuir 14,3971 (1998) and Langmuir 16, 9944 (2000); M. A. Markowitz et al., Appl.Biochem. and Biotechnol. 68, 57 (1997). Hydrogels are mainly composed ofwater, and water easily diffuses through a hydrogel matrix. Thus,hydrogels avoid hydration problems associated with PDMS based printing.However, hydrogel stamps change volume on exposure to water or upondrying. Also, molecules can diffuse between protrusions of the stamp.Hydrogel stamps for parallel printing of different molecules with goodregistry and separation among the spots have yet to be demonstrated.

Printing of biological molecules from an affinity stamp with catalystsand of hydrophilic molecules from a hydrophilized PDMS stamp onto asubstrate have both been demonstrated on a research level but are moredifficult to implement commercially where areal transfer over largesurfaces is desired. The difficulty arises either because there is notenough of the third medium required for hydration chemisorption, orhybridization on the substrate or because there is too much thirdmedium, preventing intimate contact and transfer. Third medium herein isthe general expression for a medium in which other components arecarried. Depending on the application, the third medium may be a gas,water, solvent, or polymer. See, for example, A. Bernard et al.,“Affinity capture of proteins from solution and their dissociation bycontact printing”, Nature Biotechnol. 19, 866 (2001).

A third medium in the form of damping water is found in offset printingof viscous inks onto impermeable substrates. See, for example, J. M.Adams, D. D. Faux and J. J. Rieber, “Printing Technology 4th Ed.”Delamare Publishers, Albany, N.Y., 1996. Offset printing typicallyemploys a printing cylinder having a rubber printing surface. Prior toapplication of ink, the surface is moistened. This transfers a thinlayer of detergent carrying water to the printing surface. The detergentreduces surface tension in the water. The water layer covers the surfacebut can be displaced by application of patterned link. A water layerimproves definition in printing processes where information on a masteris presented as a wettability pattern. The water layer preventsincursion or adherence of ink to ink repelling regions. In transfer fromthe printing surface to paper, water is absorbed into the fiber mesh ofthe paper and dried. This process does not work on impermeablematerials. In such cases, the printing rubber surface slips on the waterlayer and the pattern is smeared. A conventional solution to thisproblem is to roughen the surface to be printed and render ithydrophilic. By controlling the thickness of the water layer, fluidtransport over large areas can be prevented. This avoids need forcapillary channels that obstruct printing of pictures. Roughening alsodetermines fluid resistance in percolating channels and thereforedetermines printing speed. Roughening creates a random distribution ofpeaks and troughs. These lead to unobstructed percolation path betweenlarger zones. The random process is however inefficient because it alsocreates many disconnected capillary paths.

A third medium also affects high speed contact between a rigid objectand an adhesive tape in a gas such as air. The gas can buildconsiderable pressure between the object and the tape. The pressuredeforms the tape to create a central depression. The depression causestrapping of an air pocket. The air pocket prevents accurate positioningof the object in subsequent process steps such as pick and placeoperation in the manufacture of semiconductor subassemblies, diskread/write heads, and the like. Such assembly is increasingly importantas semiconductor technology moves from creating entire processors on onechip towards assembling sub-components on intermediate carriers. Toassemble and process several chips in parallel, in flip chip bonding forexample, typically requires pre-assembly on an adhesive tape or pad.

Self-assembly of μm-sized components on a chemically patterned surfacein a third medium is typically a slow process in which particlesapproach the target surface closely enough to allow specific molecularor chemical interactions. Typically, such a process requires vigorousagitation to provide particles with sufficient diffusion through thethird medium to establish contact with counterparts on the surface. Itcan be difficult to separate the particles when the third medium is notpresent. For assembly, it is desirable to have an intermediateinteraction between the parts to be assembled to better controlassembly. Appropriate placement produces stronger interaction, whileinappropriate placement provides produces weaker interaction. For fasterand more predictable assembly, an improved approach process formicrometer to millimeter sized particles in a third medium would bedesirable. The third medium helps suspend particles that would otherwiseaffected by gravitational forces.

SUMMARY

In accordance with the present invention, there is now provided a methodfor transferring a pattern from an elastic stamp to a substrate in thepresence of a third medium, the method comprising: controlling a layerof the third medium between the stamp and the substrate to apredetermined thickness. In an exemplary embodiment of the presentinvention, the substrate is rigid. In a particularly preferredembodiment of the present invention, the substrate is impermeable. Thethird medium may comprise one or more of gas, water, solvent, polymer,emulsion, sol-gel precursor, and the like. The controlling may compriseavoiding trapping of the third medium via the stamp matrix beingpermeable to the third medium. Alternatively, the controlling maycomprise forming a nanometer sized gap in the stamp filled with thethird medium.

The controlling preferably comprises providing a patterned stamp surfacehaving channels to drain the third medium. In a preferred embodiment ofthe present invention, the controlling comprises filling vias andrecesses formed in the stamp with a component having an affinity for thethird medium. The component may be hydrophilic. The component preferablycomprises a gel. The gel is preferably swellable by the third medium.The controlling preferably comprises swelling the gel with the thirdmedium to form protrusions in the stamp. In a particularly preferredembodiment of the present invention, the controlling comprises providingan array of protrusions and recessed zones in the stamp. The controllingmay comprise guiding excess third medium away from the surface of thestamp via the recessed zones. The array preferably comprises amicrometer-sized pattern subdivided into smaller structures. The smallerstructures may be separated by smaller drainage channels. The smallerdrainage channels are preferably connected to a network of largerdrainage channels. The third medium may be trapped in a shallowlens-like pocket between the stamp and the surface of the substrate. Thecontrolling may comprise trapping the third medium in a pocket betweenthe stamp and the substrate. The stamp may comprise channels. Thechannels define molecular sized gaps between the stamp and thesubstrate.

The present invention also extends to: use of such a method for printingbiological molecules on a surface; use of such a method for printingdyes on a surface; use of such a method for printing catalysts on asurface; use of such a method for printing acids or bases on a surface;use of such a method for printing of radical initiators on a surface;use of such a method for detection of molecules through proximity byfluorescence resonance transfer; use of such a method for purificationand concentration of reactants; use of such a method in an offsetprinting process; or use of such a method in a rolling contact process.

Viewing the present invention from another aspect, there is now provideda stamp for transferring a pattern to a substrate in the presence of athird medium, the stamp comprising a contact surface and drainagechannels formed in the contact surface.

The surface is preferably patterned. The stamp may comprise an array ofprotrusions. The patterning may comprise a micrometer sized patternsubdivided into smaller structures. The drainage channels preferablyextend between the smaller structures. The drainage channels preferablyform a network.

Viewing the present invention from yet another aspect, there is nowprovided a stamp for transferring a pattern to a substrate in thepresence of a third medium, the stamp comprising a permeable hydrophilicmatrix. The stamp may comprise active vias. The vias may be filled witha material permeable by a third medium. The stamp may additionally oralternatively comprise active recesses. The recesses may also be filledwith a material permeable by a third medium.

In a preferred embodiment of the present invention, there is provided amethod for providing controlled contact between two articles that allowstransfer with spatial control of a material from a stamp to a substratein the presence of a third medium. In a particularly preferredembodiment of the present invention, there is provided a method thatallows controlled formation of nanometer sized gaps filled with thethird medium within which molecular processes can occur. In anespecially preferred embodiment of the present invention, there isprovided a method for providing conformal or proximal contact eitherinduced by an external force or spontaneously in self-assembly. In apreferred embodiment of the present invention, there is provided amethod wherein controlled proximity of an article to a substrateproduces patterning of the surface with biomolecules or other molecules.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Preferred embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings inwhich:

FIG. 1A is a side view of a stamp approaching a slider bar and anintervening viscous polymer;

FIG. 1B to 1E are graphs showing pressure as a function of lateralposition of the stamp and gap height;

FIGS. 2A to 2B are side views of a stamp approaching a substrate in thepresence of a third medium in liquid form;

FIG. 2C shows photographs of the stamp in contact with the substrate;

FIGS. 3A to 3H are photographs of interference fringes produced byquadrilateral patterns on the stamp when in contact with the substrate;

FIGS. 4A to 4D are cross sectional views of a stamp embodying thepresent invention;

FIGS. 5A to 5F are cross-sectional and plan views of another stampembodying the present invention;

FIGS. 6A to 6C are cross sectional views of an adhesion sensor;

FIGS. 7A to 7D are cross sectional and plan views of yet another stampembodying the present invention;

FIGS. 8A and 8B are cross sectional views of stamps having shallowchannels;

FIG. 9 is a cross-sectional view of a bonding pad embodying the presentinvention;

FIG. 10A is a cross sectional views of a printing cylinder;

FIG. 10B is a cross sectional view of a printing cylinder embodying thepresent invention;

FIG. 11A is a side view of a printing cylinder;

FIG. 11B is a side view of a printing cylinder embodying the presentinvention;

FIG. 12A is a block diagram showing spontaneous interaction between aparticle and a flat surface with patterning; and,

FIG. 12B is a block diagram showing spontaneous interaction between aparticle and a flat surface without patterning;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Problems associated with printing from a stamp to a solid impermeablesubstrate by conformal or proximity contact can stem from an excess of athird medium such as a solvent. The excess prevents intimate contact andtransfer because it forms a gap between the stamp and the substrate. Thegap is filled with the excess thus preventing conformal contact.Problems can also arise if there is a lack of third medium on thesubstrate. Hydration, chemisorption and/or hybridization on thesubstrate can then be adversely affected. For example, biomolecular andchemical reactions usually require a third medium such as a solvent tofunction. In the first case, it is desirable to control the amount ofthe third medium to a well defined layer thickness. In a preferredembodiment of the present invention, this is achieved by providingdrainage channels in the stamp surface. In the second case, it isdesirable to offer a controlled amount of the third medium to thesubstrate. In a preferred embodiment of the present invention, this isachieved via a permeable stamp matrix.

The physics underlying printing in the presence of a third medium can befurther understood by considering a flat stamp approaching a flatsurface. The relation between gap height h and pressure p in compressedmedium of viscosity η is described by Reynold's equation. See, forexample, A. Cameron, “Basic Lubrication Theory”, Wiley (New York 1981),Chapter 3.7. In the following example, a one-dimensional model is used.Reynold's equation thus simplifies to:

$\begin{matrix}{{\frac{\mathbb{d}}{\mathbb{d}x}\left( {{\frac{h^{3}}{\eta} \cdot \frac{\mathbb{d}}{\mathbb{d}x}}p} \right)}:={{12 \cdot \frac{\mathbb{d}}{\mathbb{d}t}}h}} & (1)\end{matrix}$where x is the coordinate parallel to the surfaces and t is time. Themodel can be applied to elongate surfaces. Examples in the data storagefield include thin film head sliders. Typical dimensions of such slidersare 1.2 mm.times.50 mm. For squarer geometry, pressure may be reduced bya factor of around 2.

If both stamp and surface are rigid, h is independent from x. Thedifferential equations for p and h can be solved. Thus, the origin ischosen in the middle of the surface of width w.

$\begin{matrix}{{p(x)}:={\frac{3}{2} \cdot P \cdot \left\lbrack {1 - \left( \frac{2 \cdot x}{w} \right)^{2}} \right\rbrack}} & (2)\end{matrix}$

FIG. 1A is a cross section of a stamp 2 and slider 4 with an interveningthird medium 8. FIG. 1B shows that the pressure profile has a parabolicshape having a maximum in the center and dropping to zero at the edges.The maximum is 1.5 times the mean pressure P.

In practical implementations, either the stamp 2 or the surface 4 iselastic. The aforementioned pressure distribution causes a concaveelastic deformation in the elastic part. This can lead to pocketstrapping the third medium during contact. These trapped areas of thethird medium are referred to as “pancakes”. The normal deformation canbe calculated from the pressure distribution based on a formula derivedby Bietsch and Michel in “Conformal contact and pattern stability ofstamps used for soft lithography” J. Appl. Phys. 88, 4310 (2000). For aslider geometry, a mean pressure of 1 bar can lead to concavedepressions up to 10 μm in a typical silicone elastomer having a Young'smodulus of 3 MPa. The deformation scales with the Young's modulus. Aharder material reduces pancakes. In the case of slider processing, theslider 4 is rigid and the stamp 2 is elastic.

The pressure and gap height during the approach are closely related.There are two cases. The first case is constant applied load. The secondcase is constant speed of approach.

When constant load is applied, there is a constant pressure distributionaccording to equation 2. P is the mean pressure acting on the surface 4.The gap height is then calculated from equation 1:

$\begin{matrix}{h:=\sqrt{\frac{n \cdot w^{2}}{2 \cdot P \cdot t}}} & (3)\end{matrix}$

The calculations provide an estimate of how fast the third medium 8 isdisplaced by the stamp 2. The third medium may be a viscous prepolymer,gas, water, or a solvent. FIG. 1E shows decrease of gap height fordifferent fluids of viscosities 100 cP, 1000 cP and 10000 cP at 1 barapplied load as a function of time. The viscosities are typical forUV-cureable polymeric materials. A gap as small as 1 μm is achieved for100 cP within 1 s. However, a time of 100 s is required for the higherviscous material of 10000 cP.

In the second case of constant approaching speed, there is increasingpressure when the gap height is decreasing. This effect depends onviscosity, the speed (v) and the dimension of the punch.

$\begin{matrix}{p:=\frac{4 \cdot n \cdot v \cdot w^{2}}{h^{3}}} & (4)\end{matrix}$where p is the mean pressure. FIG. 1C shows pressure as a function ofgap height for a slider, when the third medium is water and the speed ofapproach is 10 μm/s. If the third medium is air, having a 60 times lowerviscosity, this diagram is true for a speed of 600 μm/s. In thisexample, the pressure increases from moderate values (100 Pa) for a gapwidth “w” of 10 μm to values greater than 105 Pa (=1 bar) when the gapis reduced below 1 μm.

FIG. 1D shows how the pressure depends on the dimension of the surface4. At a gap width of 100 nm the pressure is reduced to 50 Pascal for awidth of the surface 4 of 1 μm compared to 50 MPa for a typical slidergeometry of 1.2 mm. The maximal pressure scales with the inverse squareof the stamp or surface size.

According to Bietsch and Michel, “Conformal contact and patternstability of stamps used for soft lithography” J. Appl. Phys. 88, 4310(2000), stamps deform under constant pressure to form so called“sagging” profiles. FIG. 2A shows an elastomeric stamp 10 approaching arigid surface 12 of a substrate 14 in a third medium 16, such as water.The third medium 16 is displaced between the protruding features 18 ofthe stamp 10 and the substrate 14. See the arrows in FIG. 2A. When thegap between the stamp 10 and the substrate 14 becomes very small, thethird medium 16 cannot be displaced instantly. Pressure thus builds upwith a maximum below the center of the features 18. See the inset ofFIG. 2A. The pressure build up elastically deforms the surface 20 of thefeatures 18. When the stamp 10 contacts with the substrate 14, alens-like pocket 21 of the third medium is trapped below each feature18. See FIG. 2B. The profile follows the pressure distribution in thesqueezed third medium 16. See again FIGS. 1B and 2A. FIG. 2C showsphotographs of pancakes of water between an elastic hydrophilic stampand a rigid glass surface 22. In this example, the stamp has squareprotrusions 24 of size 200 μm molded in Sylgard® 184. The stamp waspressed onto the surface 22 with a relatively low pressure ofapproximately 0.05 bar. This produces a load of 5000 Pa× fill factor,where fill factor is represented by the contact area divided by theoverall area. Interference fringes in the form of Newton rings 24 arepresent. From the Newton rings 24 and the measured refractive index of1.3, a maximal thickness of 350 nm of enclosed water 16 was estimated.The weak definition of the Newton rings 24 did not allow exactdetermination of the thickness as a function of protrusion size. Todemonstrate the effect with more definition, experiments with UV-curablepre polymers as a third medium were conducted. The results of theseexperiments are summarized in FIGS. 3A to 3H. FIGS. 3A to 3H showphotographs of interference fringes on quadratic patterns having sizesof <20, 20, 60, 100, and 200 μm, where the term “60/20 microns”describes the width of features in μm. In FIGS. 3E and 3G, deformationof elastomeric protrusions is measured. FIGS. 3E and 3G show the samestructures as FIGS. 3F and 3H but with a larger view. Thickness analysisas a function of pattern size showed a linear size dependence having anintercept close to 0 and a slope of 4 nm per μm pattern size. This showsthat the enclosed layer thickness linearly scales with pattern size. Acomparison of trapped third medium and the layer on the 200 μm squareprotrusions shows a difference of a factor 3. This is attributed to theviscosity difference between water and the UV-curable prepolymer. Basedon these results, a stamp for direct contact with no water (e.g., alayer <1 nm) should have patterns with sizes smaller than 1000 nm. Onthe other hand, it is also possible to choose larger features to creategaps with defined thickness. Based on these results, a controlled layertransfer over a gap of 4 nm, for example, can be performed by selectingprotrusions with 3 μm size, and a pattern transfer over 20 nm can beperformed by selecting protrusions with 250 μm size.

Flow resistance in fluidic networks scales with the inverse of smallestchannel dimension and with channel length. Capillary force also scaleswith the inverse of channel dimensions. Fluid mechanics allows scalingof networks to nanometer dimensions. However, patterning with thesenetworks is restricted by surface to volume ratio. A large surfacepermits molecules dissolved in a liquid to encounter the surface andreact accordingly. This leads to depletion of the liquid. Capillarynetworks are therefore very efficient in patterning via relatively shortchannels with channel dimensions in the micrometer regime. See, forexample, Delamarche et al., “Microfluidic networks for chemicalpatterning of substrates: Design and application to bioassays”, J. Am.Chem. Soc. 120, 500 (1998). When dimensions are in the nanometer scale,molecules are preferably brought to desired locations by other means.However, networks can still guide fluid to and from different zones. Inimmersed systems with no liquid/air interface present, capillary forcesare immaterial. In this case, flow resistance is approximatelyproportional to the product of channel length and inverse normalizedchannel dimension, (w+h/(w*h))², where w is the width and h is theheight of the channel. Scaling branched fluidic networks to thenanometer scale involves channels with different orders of magnitude:channels with small dimensions for short paths; channels with mediumdimensions for intermediate paths; and, channels with large dimensionsfor long paths. Combining two or three layers of appropriately sizedchannels allows guidance of fluids from macroscopic to nanoscopicdimensions in a perfusion system or from nanoscopic to macroscopicdimensions in a drainage system. This is similar to the human bloodcirculation system, in which several nested subsystems are used to scalefrom meters in arteries with pumped flow to nanometers in cell gaps.

Efficient biological printing and catalytic conversion involvesdefinition and control of a thin layer of solvent between stamp andsubstrate. This is not however physically stable in conventionalsystems. See, for example, A. Martin et al., “Dewetting nucleationcenters at soft interfaces”, Langmuir 17, 6553 (2001), describing thespontaneous dewetting of a meta stable liquid film on an elastomericsurface. In a first embodiment of the present invention, this problem issolved by avoiding the unwanted trapping of a third medium via apermeable stamp matrix. In a second embodiment of the present invention,this problem is solved by providing a patterned stamp surface thatcontrols the thickness of the third medium layer and allows excessmedium to escape through drainage channels.

In an example of the second embodiment, a layer of third medium istrapped between a protrusion of the stamp and the substrate. The thirdmedium is used to carry out deposition of molecules and to provide anenvironment for catalytic reactions. In another example of the secondembodiment, patterned stamp surfaces are provided in which recessesdefine molecular sized gaps. The gaps allow transfer of DNA oligomersand polymerase chain reactions (PCR) at desired locations. In both theseexamples, the target substrate is preferably within the length of themolecule to facilitate efficient interaction.

Referring now to FIG. 4A, in an example of the first embodiment, a stamp26 comprises active vias 28 and recesses 30. Referring to FIG. 4B, thevias 28 and recesses 30 are filled with a polymer gel matrix 32permeable by a third medium, such as water or other buffer material.Plugs are thus formed in the vias 28 and recesses 30. By uptake of thethird medium, the gel 32 swells to an equilibrium state such that thegel 32 protrudes beyond the surface 33 of the stamp 26. The swilling maybe performed in a 100% vapor phase environment. The stamp 26 may then bestored in such an environment to prevent subsequent drying of the gel32. Because the gel 32 within the stamp 26 is held in another material,the metrology of the stamp 26 is not affected by the swelling. Referringto FIG. 4C, and particularly the arrows therein, the protrusions of gel32 are then selectively addressed via a stencil 34 and filled with themolecules for patterning. Each via 28 and recess 30 may be loaded withdifferent molecules to be transferred. Because the plugs of gel 32 inthe vias 28 and recesses 30 are isolated, there is no interdiffusionbetween neighboring plugs. With a via thickness of 10 μm and a loadingwith 1-weight-percent of molecules, the amount of material stored in thestamp 26 is sufficient to print several hundred monolayers of molecules.Referring to FIG. 4D, the stamp 26 is now brought into contact with asubstrate 36 to transfer the desired amount of material. The stamp 26need not be immersed in liquid, thus reducing printing complexity. Thegel 32 holding the third medium provides full solvatation of themolecules and also a good environment for a chemisorption reaction. Thepermeability of the gel 32 allows any third medium trapped between thestamp 26 and the substrate 36 to escape through the gel 32. This avoidsseparation of stamp 26 and substrate 36 by the third medium hiking ofthe different vias 28 and recesses 30 may be performed via sequentialmethods such as pipetting, pin spotting, or ink jet spotting.Semi-parallel methods based on fluidic networks may also be used toprovide selective addressing. Other examples of this embodiment maycomprise only vias 28. Similarly, further examples of this embodimentmay comprise only recesses 30. There is no additional layer of thirdmedium trapped between stamp 26 and substrate 36. However, the thirdmedium can be in contact with the substrate 36 as a majority componentof the gel 32. Thus, the gap between stamp 26 and the substrate 36 neednot be controlled. Another application of stamps with gel protrusionspermeable by a third medium and not fully swelled is the combinedconcentrating and printing of diluted solutions. This is generallyuseful for detection of molecules and particularly useful for detectionof pollutants at extremely low concentrations. Examples of pollutantsinclude metal ions such as Pb.sup.2+, Hg.sup.2+, Zn.sup.2+, etc.Detection can then be achieved by measuring adhesion during removal.Other detection schemes are possible.

Referring to FIG. 5A, in an example of the second embodiment, ahydrophilized elastomeric stamp 38 has an array of protrusions 46 with ahigh fill factor. Each protrusion 46 is subdivided into smallerprotrusions 40 separated by recesses 42 acting as small channels toguide excess third medium away before printing contact with thehydrophilic surface 50 of a substrate 51 is established. Referring toFIG. 5B, the smaller protrusions 40 can be circular, rectangular, or ofother cross section, in square, hexagonal, or other packing. The contactarea of the smaller protrusions 40 is maximized while simultaneouslyleaving the smaller channels 42 to form an open linked network. Thelarger protrusions 46 are separated by larger drainage channels 48 incommunication with the smaller drainage channels. In a preferredexample, the protrusions 40 have a size of 10 μm and a height of 3 μm.Other dimensions are possible. FIG. 5C shows protrusions 40 approach thesubstrate 51 in the presence of the third medium 41. FIG. 5D shows localtrapping of third medium 41 in shallow pockets between the protrusions40 and the substrate 51. The size of the pockets 52 may be 80% of thatof the protrusions 40. The depth of the pockets 52 is proportional tothe square of the size of the protrusions 40. FIG. 5E shows molecules 43attached to the substrate 51 and to the protrusion 40 within one of thepockets. FIG. 5F shows interaction between the molecules 43 attached tothe substrate 51 and the molecules 43 attached to the protrusion 40within one of the pockets. For molecular transfer and controlledexecution of a chemisorption reaction, a gap between the stamp 38 andthe surface 50 of the order of 2 nm is usually sufficient. According toan experimentally determined ratio of 750 between protrusion size andgap thickness, patterns having a size of 1.5 μm is suitable. Therecesses 42 providing the drainage channels on the stamp 38 are mutuallyconnected to drain the third medium away into the larger channels 48.Different protrusions 40 on the stamp 38 can transfer differentmolecules by selective inking Sequential methods such as roboticpipetting, pin spotting, or ink jet spotting may employed for suchinking. Semi-parallel methods related to fluidic networks are equallyapplicable.

FIG. 6 shows adhesion force between stamp 101 and substrate 102 in athird medium 103 as a function of stamp substructures. Referring to FIG.6A, large surfaces do not trigger noticeable molecular interactions.Referring to FIG. 6B, medium (10 μm) protrusions 104 trigger smallinteraction forces. Referring to FIG. 6C, small (<10 μm) protrusions 105show strong interactions.

Referring now to FIG. 7A, in another example of the second embodiment, astamp 52 has shallow elongate parallel channels 54 formed therein. Thechannels 54 are separated by intervening walls 53. In preparation foroperation, the channels 54 are coated with particular molecules 56. Inoperation, the channels 54 form active zones in which molecules 56 onthe stamps 52 are brought into proximity with molecules 56 on thesurface 58 of a substrate 60 when the stamp 52 is brought into contactwith the substrate 60. The molecules 56 on the stamp 52 and thesubstrate 60 interact within each channel 54 when the stamp 52 is incontact with the substrate 60 in presence of a third medium 62. Forbiomolecular interactions, the third medium 62 may be water or a waterbased solution containing other solvents, buffer ions, nucleosidesand/or enzymes. The channels 54 define a layer of the third medium 62with sufficient thickness to allow performance of a biochemical process.The stamp 52 is preferably made from a thin layer of elastic material toprovide large area molecular contact. An externally applied load isapplied such that any sagging induced is sufficiently small to provide asubstantially uniform gap thickness in each depression 54. The load canbe regulated to adjust the gap. Referring to FIG. 7B, if the load is toosmall, the gap may be too large to permit interaction between themolecules on the stamp 52 and molecules on the substrate 60. Similarly,referring to FIG. 7C, if the load is too large, the stamp 60 maycollapse and gap may be too small to permit interaction between themolecules on the stamp 52 and molecules on the substrate 60. Referringto FIG. 7D, in a particularly preferred example, a stamp 60 is patternedvia contact lithography for producing a 25 mm biochip. The channels 54are 4 μm wide and the separating walls 53 are 60 μm long, 1 μm wide, and25 nm high. Excess third medium 62 displaced during printing iscollected by 40-μm-wide and 40-μm-deep drainage channels 57 and drainedaway on a macroscopic scale by several millimeters. The drainagechannels 57 define active zones of the stamp occupied by groups of thesmaller channels 54. The desired channel height depends on the moleculesinvolved and can vary from e.g., 2 nm to 200 nm. In general, if themolecular length is 20 nm then a channel of >20 nm is too large and achannel of <5 nm is too small. The drainage channels 57 permit printingof a relatively large substrate 60 without limiting effective fillfactor. The stamp 52 may be molded in Sylgard® 184 from a master with acompression modulus of 3 Mpa. The master may be fabricated vialithography methods such as projection lithography and e-beamlithography. Such a stamp 52 may be pressed onto the surface 58 with anaverage pressure of around 3 kPa distributed over the area of the stamp52. In this example, drainage may require around 10 seconds, duringwhich the larger channels guide 57 away the third medium over around 70mm. FIG. 8A shows the height profile across a 4-μm-wide and initially 25nm high channel 54 molded in Sylgard® 184 with a compression modulus of3 Mpa. The channel 54 was placed in contact with a substrate 60 under3000 Pa pressure. The channel 54 is compressed to 22 nm at the edges andto 18 nm in the center. A±10% gap width accuracy is achieved. This isconsistent with the length tolerance for hybridization on oligomers. Totune the stamp 52 to a different system of molecular interaction, thewidth of the channel 54 can be adjusted by changing the load. Areduction of the load to 1500 Pa, for example, increases the minimalchannel width from 18 to 22 nm. In a second example, the stamp 52comprises 10.times.10 μm sized active zones each having 6 12 μm long 200nm wide and 25 nm high supporting walls 53 separating 1800 nm wideshallow channels 54. 8 μm deep and 8 μm wide drainage channels 57 directexcess third medium such as water to the boundary of the stamp 52around±10 mm away. The stamp 52 may again be molded from Sylgard® 184with a Young's modulus of 3 MPa and pressed onto the surface with anaverage pressure of 5 kPa distributed over the stamp 52. The time neededto displace excess water to the boundary with the selected pressure maybe again around 10 seconds. FIG. 8B shows a height profile across such a1800 nm wide and initially 25 nm high channel 54. In some embodiments ofthe present invention, materials other than Sylgard® 184 and possiblyharder may be used.

Referring now to FIG. 9, in another embodiment of the present invention,a bonding pad 68 comprises flat elastomeric adhesive protrusions 70separated by drainage channels 72. The channels 72 permit a third mediumsuch as air to escape when a flat object 74 is placed on the pad 68 athigh speed. At high speed, a pressure of greater than 1 bar builds up inthe third medium at a gap height of 0.2 μm or more. The protrusions 70extend from an elastomeric layer 76 supported by a backplane 78. Theelastomeric layer 76 may be a siloxane rubber such aspoly(dimethylsiloxane). This material relaxes to its original shapeafter release of mechanical stress. The natural adhesive proprieties ofthe surface may be enhanced with adhesives or other surface activation.The backplane 78 is a flat layer such as thin glass, metal, silicon orpolymer, holding the elastomeric layer 76 accurately in place andpreventing lateral and vertical distortions. The pad 68 accurately holdsthe parts 74 in place in a coplanar fashion to allow accurate robotictransfer of the parts 74 to a carrier substrate or to allow parallelprocessing of the parts 74. Removal of the parts 74 from the pad 68 istypically performed by peeling to avoid potential overloading of theparts 74 or the pad 68. Such overloading may occur in other separationtechniques such as, for example, vertical pulling. An exampleapplication is thin film head slider fabrication. Present sliders 74have typical dimensions of 1.times.1 mm.sup.2 and can be accuratelyplaced onto a substrate by robot at vertical speeds of 10 mm/s. Accurateresults are achieved with elastomeric protrusions 70 of 10 to 20 μmwidth and separated by drainage channels 76 of typically 1-5 μmdiameter. The channels 76 prevent trapping of air pockets between thesliders 74 and the pad 76 by allowing air to escape. Air pressureremains moderate, exceeding 1 bar only at distances closer than 150 nm.Stamp deformation is low. The trapped air is negligible and any residualair can be quickly dissipated through the pad 68. Elastomeric siliconerubbers are surprisingly permeable for small amounts of gases. Withoutthe drainage channels 76, a pressure of greater than 1 bar builds upwhen a slider 74 approaches the pad 68 closer than 2 μm. Air pockets arethen trapped under the slider 74. The air pockets distort the pad 68 inan unpredictable way and create vertical and/or lateral distortions.

Controlled layers of water are important in offset printing processesfor reliable print contrast. Dedicated topographic patterns improvecontrol over ink buffering, water buffering and tangential transport ofliquid. To maintain print contrast, it is desirable to avoid longerdistance net transport. FIG. 10A shows an side view of a typical surface80 of a printing cylinder 82 with random roughening. FIG. 10B is asection through a micro structured surface 84 in which percolation paths86 are disposed. With the advent of low-cost micro structuring, printingprocesses can be made more efficient by exchanging random roughening forwell defined structures. The well defined micro structures optimizetangential and axial flow without reducing fill factor. This, isespecially important for printing operations onto impermeable surfacessuch as metal, glass, or ceramic where excess liquid cannot penetrate orotherwise escape the printing gap.

Topographic patterns like those herein before described provide improvedcontrol over ink buffering, water buffering and tangential transport ofliquid. In a preferred embodiment of the present invention, smalltrenches are formed in one of the contact surfaces to create a connectedfluid mesh. The mesh permits high printing speeds, allows largerparameter ranges for inking, allows thicker printed layers, reducesdependency of color mixing on printed patterns, and simplifies dampingand inking. Topographic patterns for controlled water flow are importantin flat printing such as biochip patterning and also in printing fromstamps wrapped onto cylinders to form rolling contacts. FIG. 11A showsflow resistance of fluid escaping a smooth advancing cylinder 88. Theresistance is represented by the large arrow 90. This resistancegenerates pressure. The pressure lifts the cylinder 88 in a similarmanner to aqua planing of car tires. FIG. 11B shows that an advancingcylinder 92 having a circumferentially disposed drainage pattern 94creates a smaller pressure in the liquid as indicated by the smallerarrow 96. The cylinder 92 is thus less susceptible to aqua planing.Thus, faster printing speeds can be achieved. In a rolling contact, thethird medium is displaced ahead of the cylinder or laterally if there isexcess medium only partially along the cylinder. The right half of FIG.11 shows the gap between the cylinder surface and the substrate 89 as afunction of the distance from the cylinder axis and its tangentialapproximation. In FIG. 11A, the fluid resistance is high because theremaining gap is small. In FIG. 11B, the fluid resistance is smallerbecause of channels 94 formed in the surface of the cylinder 92.

In another embodiment of the present invention, self-assembly ofmicrometer sized particles 110 using specific molecular interactioninvolves patterning of either a substrate surface 111 or contactingsurface 112 of the particles 110 with structures herein beforedescribed. The patterned contact surfaces 112 improve the binding speed.In addition, the patterned contact surfaces 112 allow faster separationof unbound or partially bound particles 110. This improves overall speedof self-assembly process and improves specificity of interaction overparticles 113 with no patterned surface 112. FIG. 12A shows particles110 having a patterned surface 112 interacting specifically with asurface 111. In FIG. 12B, there is shown a slower and less specificinteraction between the surface 111 and particles 113 without apatterned surface. The receiving surface 111 can be patterned instead ofthe particles 110.

1. An apparatus for transferring a pattern to a substrate in thepresence of a third medium, comprising: a stamp comprising a pluralityof first protrusions separated from one another by first drainagechannels; each first protrusion further being subdivided into aplurality of smaller second protrusions separated from one another bysmaller second drainage channels, with both the first and seconddrainage channels configured to guide excess portions of the thirdmedium away from a contact surface of the stamp; and wherein theplurality of smaller protrusions are arranged in a hexagonal packingarrangement.
 2. The apparatus according to claim 1, wherein theplurality of smaller protrusions have a circular cross section.
 3. Theapparatus according to claim 1, wherein the second drainage channelsform an open linked network.